COVID-19 deaths in the U.S. have climbed to an average of more than 1,900 a day for the first time since early March, with experts saying the virus is preying largely on a distinct group: 71 million unvaccinated Americans.

The increasingly lethal turn has filled hospitals, complicated the start of the school year, delayed the return to offices and demoralized health care workers.

Twenty-two people died in one week alone at CoxHealth hospitals in the Springfield-Branson area, a level almost as high as that of all of Chicago. West Virginia has had more deaths in the first three weeks of September – 340 – than in the previous three months combined. Georgia is averaging 125 dead per day, more than California or other more populous states.

The nation was stunned back in December when it was witnessing 3,000 deaths a day. But that was when almost no one was vaccinated.

Now, nearly 64 percent of the U.S. population has received at least one dose of the COVID-19 vaccine. And yet, average deaths per day have climbed 40 percent over the past two weeks, from 1,387 to 1,947, according to data from Johns Hopkins University.

Health experts say the vast majority of the hospitalized and dead have been unvaccinated. While some vaccinated people have suffered breakthrough infections, those tend to be mild.

The number of vaccine-eligible Americans who have yet to get a shot has been put at more than 70 million.

"There is a very real risk you'll end up in the hospital or even in the obituary pages," Dr. Bruce Vanderhoff, chief medical officer for the Ohio Department of Health, said to the unvaccinated. "Don't become a statistic when there is a simple, safe and effective alternative to go out today and get vaccinated."

Many low-vaccination communities also have high rates of conditions like obesity and diabetes, said Dr. William Moss of Johns Hopkins. And that combination – along with the more contagious delta variant – has proved lethal.

(With input from AP)